This invention relates to electrostatically operated microelectromechanical systems (MEMS) devices involving integrated dielectric layers, particularly capacitive switches and more particularly, to radio frequency (RF) microelectromechanical systems (MEMS) capacitive switches.
RF-MEMS capacitive switches have many useful applications for military and commercial RF and microwave applications. A RF-MEMS capacitive switch comprises a movable metal membrane (upper electrode) suspended above a lower electrode and interposing dielectric layer. An alternative configuration for RF-MEMS switches includes a separate upper electrode (which can be used as a pull-up electrode) positioned above the membrane and physically separated from the latter. An air gap of several microns typically separates the upper membrane from the dielectric layer. The lower electrode comprises a RF signal path, while the upper electrode, whether separate or being the membrane) comprises a RF and DC ground. In the switch “off state”, the air gap between the membrane and lower electrode is sufficient that the upper membrane has an insignificant parasitic capacitance relative to the operating frequency of the switch. When a voltage is applied across the upper and lower electrodes, the electrostatic force pulls the membrane down into contact with the dielectric layer (“on state”). Without a significant air gap, the upper metal membrane, insulator layer, and lower metal electrode form an MIM (metal-insulator-metal) capacitor. This capacitor is designed to achieve sufficient capacitive conductance such that it can capacitively couple, or even short, the RF signal path of the lower electrode to the grounded upper metal membrane. When the applied voltage is released, the restoring force of the membrane metal spring is sufficient to return the membrane to its “off state”, if no secondary effects impede that action, such as charging of the dielectric layer on top of the bottom electrode, and/or force of adhesion between the membrane and the dielectric layer, due to the chemical state (hydrophilic nature of the surface or water adsorption capability-high hydrophilicity may result in strong force of adhesion between two surfaces) of the surface of the membrane and/or the dielectric layer
RF-microelectromechanical switches (RF-MEMS switches) provide many potential benefits over conventional semiconductor-based switches for controlling and routing microwave and millimeter-wave signals. RF-MEMS switches possess very low insertion loss, miniscule power consumption, and ultrahigh linearity. These characteristics make RF-MEMS switches ideal candidates for incorporation into passive circuits, such as phase shifters or tunable filters, for implementation in communications and radar systems at RF, microwave, and millimeter-wave frequencies (10 MHz-100 GHz and up).
Despite the excellent RF performance of these devices, their insertion into military and/or commercial high frequency systems has been limited by a lack of reliability. In a well-engineered RF-MEMS switch, dielectric charging in the dielectric layer positioned on top of the bottom electrode is the main limitation to lifetime, as opposed to mechanical effects. When the switch actuates, a relatively high voltage (30-50 volts) is applied across the relatively thin insulator-dielectric layer on top of the bottom electrode. The resulting electric field induces charge (electrons) tunneling into the insulator-dielectric layer, where they are trapped. As these charges build up, they shift the pull-in and release voltages of the switch. If enough charges become trapped, the operating voltages will shift sufficiently such that the switch will either remain stuck down, or not actuate when desired. In either case, the switch fails to operate properly.
Furthermore, while the RF performance of these devices can be appropriate for a commercial device, from the electromagnetic signal transport point of view, reliability issues have limited their deployment into military and commercial low and high-frequency RF systems. In the case of capacitive RF-MEMS switches, shortcomings relating to dielectric charging have been difficult to mitigate. There are many solutions for lessening the impact of dielectric charging, including hermetic packaging, minimizing the electric field across the dielectric layer, and tailoring the polarity and waveform of bias control signals to minimize charging. These solutions have provided significant improvements in reliability, but have not proven enough to overcome the “stigma” associated with dielectric charging.
Commercially available RF-MEMS switches use silicon dioxide (SiO2) or silicon nitride (Si3N4) as a dielectric layer material in a capacitive switch. During operation, charges become trapped in the dielectric layer building-up over time. As the charge builds-up, the operation of the device degrades until it fails. In fact, it fails very slowly. Studies have shown that the charge and discharge time constants for SiO2 and silicon nitride Si3N4 dielectric layers are on the order of 10 s of seconds to 100 s of seconds. After failure, a device may take days to recover because charges trapped in the dielectric layer take a long time (100 s of seconds) to be transported to and be neutralized at the metal electrodes of the capacitor. The amount of charge accumulated is exponentially related to the applied electric field. The higher the operating voltages, the longer the switches are left in the “on state”. Furthermore, the higher the operating temperature, generally the faster the switch will fail.
More specifically, prior art RF-MEMS capacitive switches with oxide or nitride dielectric layers are designed in such a way that the charges accumulate as slowly as possible. Prior switches slowly degrade until the point of failure. Switches with oxide or nitride dielectrics possess inherently long discharging time constants. Charging and discharging time constants are approximately equal. Therefore, once failure has occurred, conventional prior art devices are not available for proper operation for a very long time period, rendering the device essentially useless for a majority of applications and uses.
The primary failure mode of conventional prior art RF-MEMS capacitive switches is accumulation of charges (electrons) within the insulator layer made of silicon oxide or silicon nitride materials of the switch, in which charges tunnel into and become trapped within the dielectric. The conventional prior art RF-MEMS capacitive switch only recovers from this failure after a sufficiently long period of time (hours to days) during which the trapped charges can diffuse or migrate back to the metal electrodes.
Several techniques have been developed to mitigate the effects of dielectric charging on switch reliability, such as minimizing the operating conditions that lead to dielectric charging. For example, increased switching on-time or high operating voltage, and/or temperature result in less reliable operation. However, the designer does not often have control over these parameters. Alternatively, design modifications can be made to the switch to enable reliable operation. One alternative is to minimize the amount of dielectric material within the switch to form a mechanical support of the membrane layer. The dielectric insulating material is patterned into “posts”, which support the membrane, but minimize the amount of contact between the dielectric and the membrane, instead of a metal-insulator-metal capacitive switch, it is more properly described as a metal-air-metal switch. This modification trades capacitance ratio (ratio of on-capacitance to off-capacitance) for improved reliability.
An alternative method of reducing dielectric charging is to engineer the chemical or microstructural makeup of the dielectric layer such that it has tailored conductivity, while still maintaining good dielectric behavior. The layer with tailored electrical conductivity is defined as “fast discharge dielectric layer” for the purpose of this invention, and this term will be used heretofore in this patent. Given sufficient conduction within the dielectric, the trapped charges will have more opportunity to be carried away into the electrode, and thereby be eliminated from the dielectric layer. However, depending on the physics of the charging and discharging mechanism, the quiescent current may not always be the proper mechanism for causing the induced charges to dissipate, in which the quiescent current provides no substantial advantage.
There have been attempts to manipulate the bulk conductivity of the dielectric film to bleed off charges and improve reliability. Unfortunately, these techniques have not proven repeatable or sufficient enough to be generally adopted.
It is, therefore, desirable to provide an improved reliable RF-MEMS capacitive switch, or any electrostatically operated MEMS device involving dielectric layers, which overcomes most, if not all of the disadvantages described above.